Living Legend #6 (by Elliot S! Maggin and Klaus Janson)
in
Superman #400 relates the
story of a family celebrating Miracle Monday in 5902.
Herzog Benedix and his son Riley tell the story of the
holiday:

"Across the galaxy tonight, wherever the human race has made its home,
families gather for the Miracle Monday dinner - a celebration
of our freedom!

"Let all who are hungry come and eat! Let all who are in want come ...

"Each of us now will put a portion of our food into the empty dish -
Superman's dish, which we reserve for his return to us!

"We celebrate today because thousands of years ago - Superman and
other great leaders taught us to live as a free people! Today we are
free from crime... Free from tyranny... Free from ignorance ..."

Young Riley Benedix continues the story of this festive day for his
family - it is a story of the days when America was young - and a
child who could change the course of mighty rivers came to earth to
exemplify all that America had and would come to stand for! Some of
the story is accurate - some is clouded by the folds and myths of time
- but like art and greatness, it is all true!

Celebrating Miracle Monday in 2857:

According to the Superman novel
Miracle Monday, the traditional
well-wishing greeting on Miracle Monday in 2857 is, "Good
Miracle Monday."

The novel also describes (in
chapter three) how the holiday is celebrated that year:

For Earth humans everywhere it was a special day, the third
Monday of the month: Miracle Monday.

On Miracle Monday the spirit of humanity soared free. This
Miracle Monday, like the first Miracle Monday, came in the spring of
Metropolis, and for the occasion spring weather was arranged wherever
the dominion of humanity extended. On Uranus's satellites where the
natives held an annual fog-gliding rally through the planetary rings,
private contributions even made it possible to position orbiting
fields of gravitation for spectators in free space. On Titan, oxygen
bubbles were loosed in complicated patterns to burst into flame with
the methane atmosphere and make fireworks that were visible as far as
the surface of saturn. At Nix Olympica, the eight-kilometer-high
Martian volcano, underground pressures that the Olympica Resort
Corporation had artificially accumulated during the preceding year
were unleashed in a spectacular display of molten fury for tourists
who walked around the erupting crater wearing pressurized energy
shields. At Armstrong City in the Moon's Sea of Tranquility there was
a holographic reenactment of the founding of the city in the year
2019, when on the fiftieth anniversary of his giant leap for mankind
the first man on the Moon returned, aged and venerable, to what was
then called Tranquility Base Protectorate, carrying a state charter
signed by the President of the United States. The prices of ski lift
tickets on Neptune inflated for the holiday. Teleport routes to
beaches and mountains on Earth crowded up unbelievably.
Interplanetary wilderness preserves became nearly as crowded with
people as Earth cities. Aboard the slow-moving orbital ships that
carried ores and fossil materials on slowly decaying loops toward the
sun from the asteroids, teamsters partied until they couldn't see. On
worlds without names scattered throughout this corner of the Galaxy,
where Earth's missionaries, pioneers and speculators carried their own
particular quests, it was a day for friends, family, recreation and -
if it brought happiness - reflection.

The First Miracle Monday:

The full tale of Miracle Monday and the events that
transpired on that historic day can be found in
the novel.

Shortly before four in the afternoon on the third Monday in the month
of May, the people of the city of Metropolis learned the meaning of
joy - the first thing many of them saw was the red-and-blue figure
of Superman drawing a line across their sky, and he became the symbol
of their joy. It felt like a miracle.

"Happy Miracle Monday, Superman.&nbsp It's the first Miracle Monday.
It's a holiday. People will celebrate today for hundreds, maybe
thousand of years..."